Contemporary switching mode DC to DC converters are expected to possess the virtues of high power density and high efficiency, yet at a minimum cost and with high reliability. The magnetic elements of a converter are usually the largest and most costly components, and significantly impact efficiency and reliability. Advances in the art of DC to DC power conversion often focus on techniques for reducing cost, size and complexity of the magnetic elements of converters. Of particular interest have been recent advances in the technique of integrated magnetics. Integrated magnetics allows separate transformer and inductor functions to be integrated onto a common magnetic component, thus simplifying the converter and reducing the overall number of magnetic components for increased reliability.
The usual DC to DC converter accepts energy from a low impedance voltage source and provides energy to a capacitively filtered output which functions as a voltage sink. Since the switching conversion process typically is not amenable to directly supplying a voltage sink from a voltage source, a current sink/source is imposed in the energy conversion path. This is nearly always an inductive element. For instance, the averaging inductor performs this function in the familiar Buck converter, allowing a pulse width modulated switching circuit to provide continuous regulated current flow from a low impedance voltage source input to a capacitively filtered output.
Transformers are also prominent components in DC to DC converters. Although the transformerless non-isolated Buck converter provides excellent performance for certain DC to DC converter uses, most applications are better served by a fully isolated converter. Transformers provide an ideal means for isolation, as well as the voltage level conversion so often required. For instance, the familiar Buck-derived single ended forward converter provides an isolated means for delivering continuous current from an input voltage source to a capacitively filtered output. To accomplish this, however, it requires two magnetic components: an averaging inductor, and an isolation transformer. The presence of two magnetic components burdens the converter with respect to complexity and efficiency, as compared to its non-isolated Buck counterpart, since there are now transformer windings in series with inductor windings. Furthermore, when an isolation transformer is used (such as for the single ended forward converter), transformer magnetizing flux causes transformer magnetizing current to flow in addition to the load current in the primary circuit, which increases winding and switch RMS losses due to increased RMS current flow, and can dramatically increase overall converter losses unless the energy associated with the magnetizing current can be recirculated losslessly. Contending with the magnetizing current can significantly increase cost and complexity of the converter.
Isolated DC to DC conversion with only a single magnetic element is easily accomplished by use of the familiar flyback converter. In a flyback converter, the "transformer" is, in actuality, a coupled inductor where separate windings having a turns ratio provide isolation and DC level conversion. This provides for a simpler converter than the single ended forward, since the number of output windings and cores has not increased over the non-isolated Buck converter. Moreover, the magnetizing energy in the flyback converter is actually the output energy. It is virtually all discharged through the output winding, thus avoiding the problem of the Buck derived converters where transformer magnetizing energy must be recirculated or dissipated. Unfortunately, the flyback has the drawback of discontinuous output current, and is thus only well suited for lower power applications due to output filtering requirements.
Although both the flyback and single ended forward converters effectively employ magnetic elements to provide isolated DC to DC conversion, both topologies have deficiencies in many of the other circuit parameters necessary for optimizing overall converter function. For instance, both are single ended, that is, both only operate as half-wave converters. It is well known in the art that double ended, or full wave, converters make better use of the magnetic components since the transformer is energized in both directions and energy is transferred from the primary during both switching half cycles. This improves magnetics utilization, and simplifies filtering requirements at both the input an output of the converter. A further advantage of full wave operation implemented with the bridge configurations is that it is easy to clamp the primary current associated with leakage inductance to the primary voltage source via antiparallel diodes across the switching transistors. Practical single ended designs often have the disadvantage of requiring the energy associated with the leakage inductance to be dissipated as losses in a snubber circuit. Additionally, single ended designs often have limitations for high frequency operation which adversely affect control of dV/dT (snubbing) and core reset. Full wave Buck derived converters, such as the half bridge or full bridge, overcome the high frequency operation disadvantages of half wave designs. In addition, full bridge type full wave designs can enjoy the distinct advantages of zero voltage switching techniques, such as the one taught in U.S. Pat. No. 4,864,479 to Steigerwald et al. The flyback converter, however, inherently cannot be implemented as a full wave circuit, since a half cycle of having the primary open is required for the discharging flux to provide current to the load.
In response to the challenge of making improvements in magnetics utilization, many techniques have been developed for integrating transformer and inductor magnetic functions into common magnetics assemblies. This has become known in the art of DC to DC power conversion as "integrated magnetics". Employing techniques of integrated magnetics, it is possible to combine multiple magnetic functions into a single magnetic assembly, thus simplifying the construction of conventional topologies which inherently require multiple magnetic functions for their operation. In particular, there are many ways to integrate the magnetic functions of the well-known transformer isolated derivatives of the Buck converter. Information on the design of converters employing integrated magnetics can be found in a text by Rudolf P. Severns and Gordon E. Bloom, titled "Modern DC-To-DC Switchmode power Converter Circuits" (Van Nostrand Reinhold Company, 1985).
Although implementing a conventional converter utilizing an integrated magnetic assembly can result in some improvements, further improvements have been made through the development of novel topologies which optimally utilize integrated magnetic structures. FIG. 1 illustrates such a prior art integrated magnetics converter, exemplified in U.S. Pat. No. 4,858,093 to Sturgeon. In the Sturgeon converter, a primary winding operates to induce flux in an integrated magnetic assembly so that two secondary windings provide current to a load. When the switching element energizes the primary, some current flows to the load by direct transformer coupling to the first secondary winding, while simultaneously some energy is stored in the core as an increasing flux level. When the switch terminates current flow in the primary, flux flyback action initiates current flow in the other secondary while simultaneously sustaining current flow in the first secondary. This allows energy to be effectively stored in the core while permitting continuous output current flow. Also, since the magnetizing flux in the transformer assembly provides flyback energy to sustain continuous output current flow, it does not have to be recirculated as for Buck derived isolated converters. However, the converter according to the Sturgeon patent still requires output current to flow in multiple windings in series, as opposed to just the transformer secondary winding, so there is some extra conduction loss over that which would occur if there were only one winding at a time in the output current conduction path. Moreover, since continuous current flow is sustained by flyback action, the Sturgeon converter is required to be of half-wave design. In the Sturgeon converter as well as conventional designs, half-wave operation increases stress on the switching element for a given power level as opposed to a full wave design, and precludes enjoying the further magnetics utilization and high frequency operating advantages of double ended or full wave designs.